As has been pointed out in the references of many researchers, dictionaries and patents, it takes a very long time to measure dew points and/or frost points, particularly low dew points and/or frost points, and supercooling often takes place when the dew point and/or frost point to be measured is -100.degree. C. or below.
As a result of many repeated experiments, the present inventors have discovered a method that is believed to be the most accurate way of dew point and/or frost point measurement that is practical enough to be implemented with an industrially useful dew point or frost point meter.
It has been known to determine the water content of a gas by measuring its dew or frost point (see, for example, U.S. Pat. No. 5,052,818). In that method, the gas to be measured is blown against a reflecting mirror cooled to -80.degree. C. or below and the condensation of dew or frost on the reflecting mirror is detected by a sudden increase in scattered light and the water content of the gas is determined from the dew or frost point. However, later studies of the present inventors have shown that below -90.degree. C., the amount of condensation of dew or frost on the reflector mirror is so small that the detector sometimes fails to achieve the correct sensing of the point where such dew or frost condensation has occurred. Even if dew or frost condensation can be sensed by the detector, one often cannot be sure whether the sensed point of dew or frost condensation reflects the correct dew or frost point. In addition, if a phenomenon called "supercooling" occurs, dew or frost will not be condensed at the temperature where dew or frost condensation would otherwise occur. In this case, too, one is unable to know for sure whether the measured point reflects the correct dew or frost point.
As a result of extensive research efforts, the present inventors found that when the reflector mirror was held at a low temperature even after dew or frost condensation, thereby forming the solid phase of water in a suitable amount and when a heating step, a cooling step and a heating step were conducted sequential through at least one cycle, followed by detection of the temperature where the intensity of scattered light and/or reflected light was at a maximum or a minimum and the temperature where said intensity was at a minimum or a maximum, the two temperatures were almost the same and could be regarded as the correct dew or frost point. The present invention has been accomplished on the basis of this finding.